1. Field of the Invention
The present invention relates to an apparatus for projecting the image of an original on a photosensitive member with a selected magnification using a zoom lens.
2. Description of the Prior Art
There are known systems in which the magnification of an image to be projected onto a photosensitive member will be changed by the use of a zoom lens, for example, from U.S. Pat. Nos. 3,765,760; 4,046,467; and 4,077,710 and U.K. Patent Application Nos. 2,016,732 and 2,073,899. In such systems, the magnification of the projected image is changed by moving the zoom lens with its focal length being varied. As will be well known in the art, the focal length of a zoom lens will be changed by moving at least one of the lens elements, constituting the zoom lens system, relative to at least another one of the lens elements along the optical axis of the zoom lens.
The movement of the zoom lens is different in the moving direction between when the image is enlarged and when it is reduced. On the other hand, the lens moving means and focal-length changing means are constructed by combining various mechanical components. This means that some play, backlash or the like between each adjacent components cannot be avoided. As a result, the lens element of the zoom lens will more easily create an error when it is moved in the second direction, if it is designed to move correctly in the first direction. This may provide an unclear image projected on the photosensitive member, an improper magnification and so on.
To overcome such problems, the technique described in U.S. Ser. No. 505,962 now Pat. No. 4,521,100 which is the U.S. counterpart of West Germany Patent Application No. 3322857.4 can be utilized to provide an apparatus in which a reference position is provided on the path of the zoom lens at one end and wherein if it is desired to change the magnification, the zoom lens is necessarily moved once into abutment with the above reference position and then run from the reference position to a position corresponding to the desired magnification. In such an apparatus, the zoom lens always reaches and stops at its goal position in the same direction whenever the magnification is to be changed. Accordingly, the positional errors of the zoom lens and associated zoom ring and others are equalized for any positions of the zoom lens. For this purpose, the zoom lens will be set with respect to its positions and focal length in consideration to these positional errors, so that an optical system including this zoom lens does not vary in focus condition, magnification and so on. In such an arrangement, however, the zoom lens must be moved necessarily once to its reference position on each change of magnification. Thus, it requires an increased time period for changing the magnification. For example, where the reference position of the zoom lens is at a position corresponding to its maximum magnification and if it is desired to form an image with a reduced magnification and then to form another image with another reduction smaller than the above reduced magnification only by a few percentages, the zoom lens which has been placed at a position remote from the reference position must be again returned to the same reference position and then moved to a position corresponding to the desired magnification to set the zoom lens to desired magnification. This will be time consuming when it is required to position the zoom lens relative to the desired magnification.